This invention relates to highly accurate robots. Typically, after initial joint angle calibration, the greatest source of error is compliance of the mechanical unit under loading. The greatest amount of this compliance error occurs in the gear train of the robot.
It has been recognized for many years that putting encoders on the output side of the gear train provides the capability of a highly accurate robot. U.S. Pat. No. 5,155,423 describes the encoder on the output side of the gear train: “Torque bridge 421 has mounted on its surface several strain gauges for detecting joint torque. Also, encoder 415 detects the relative rotation of the first structural assembly to the second structural assembly by monitoring the rotation of wire passage tube 438 which is communicated through the coupling represented diagrammatically at 439. The measurement of rotation is taken here so that it is independent of the dynamics of the gear train. These measurements are used for feedback control of the arm motion.”
U.S. Pat. No. 4,608,651 describes using primary and secondary encoders used for teaching/playback type of robots. This method uses the differential between the primary and secondary as a control means.
U.S. Pat. No. 6,258,007 describes a motor and reducer assembly that includes both reducer input and output encoders (see FIG. 3 from U.S. Pat. No. 6,258,007).
Traditionally, when secondary encoders are used the position loop of the control system uses the secondary encoder while the velocity loop uses the primary encoder. The pure secondary encoder position control with primary encoder velocity and torque control becomes difficult in the case of interaction between robot axes, such as an inline, three roll wrist. The control loop would need information from other axes during the low level control; this is not always feasible. Also it may not be convenient to use a single control loop at the DSP (Digital Signal Processor) level for the case of advanced calibration techniques that are required for the secondary encoder.
Using the secondary encoder as a measurement system does not provide the response required for more than low speed and stationary motion. Re-orientation or higher speed motion cannot be achieved without integration of the secondary encoders to a closed loop control system.
Also, secondary position feedback has been common in the CNC systems for years. However, this control does not combine both position control by primary encoders and position control by secondary encoders.
It is also well known that properly calibrated encoders attached to the output side of robot axes can provide an accurate position measurement system. This system is traditionally used for open loop control of position for low speed operations, such as stationary drilling.
The closest prior art to this invention is based on the following principle: Using a secondary encoder for position control while using the primary encoder for velocity control. Also, secondary encoders can be used outside the low level control system as a position measurement device to provide open loop position control.
The prior art has the following shortcomings:
The pure secondary encoder position control with primary encoder velocity and torque control becomes difficult in the case of interaction between robot axes, such as an inline, three roll wrist. The control loop would need information from other axes during the low level control; this is not always feasible.
Using the secondary encoder as a measurement system does not provide the response required for more than low speed and stationary motion. Re-orientation or higher speed motion cannot be achieved without integration of the secondary encoders to a closed loop control system.